The invention relates to a method for measuring freeness, in which method a measuring chamber comprising a wire or the like arranged on the bottom surface of the measuring chamber is filled with a suspension to be measured and the suspension is allowed to flow through the wire or the like at a time instant T0.
The invention further relates to a measuring apparatus to measure freeness, which measuring apparatus comprises a measuring chamber comprising a top lid and a bottom surface shutter that tightly seal the measuring chamber, an air valve and a wire or the like; in the beginning of the measuring process, the measuring chamber contains the suspension to be measured; the shutter of the measuring chamber bottom surface is made to be opened; the measuring apparatus is adapted to allow the suspension to flow through the wire or the like at a time instant T0.
To make good quality paper, the properties of paper stock must be precisely measured and controlled. In measuring the freeness of paper stock, the speed with which the paper stock can be separated from water is empirically determined. Freeness depends on several factors, such as fibres, stock processing (for instance mechanical/chemical), the quantity of fines, temperature, consistency, and the measuring apparatus.
One of the most common methods for measuring freeness is CSF (Canadian Standard Freeness). This measuring method is a standard and has been disclosed in detail in publication T 227 om-85, Freeness of pulp, TAPPI, 1985, which is incorporated herein by reference. In the CSF measurement, the freeness of paper stock is measured from a sample with 0.3% consistency and 20xc2x0 C. temperature. If the consistency or temperature of the sample differs from the specified values, the freeness result is adjusted according to predefined table values so that the measurement corresponds to the specified consistency and temperature values. In the beginning of the CSF measurement, exactly one litre of the sample is measured into a measuring tank comprising the walls of the tank, a top lid that closes against the top part of the walls, a wire at the bottom of the tank, a bottom lid that closes against the bottom part of the walls, and an air valve. The bottom lid is opened and the sample is allowed to settle in the tank so that some of the stock descends on the wire at the bottom of the tank. After approximately 5 s from opening the bottom lid, the air valve is opened so that water starts separating from the stock sample through the wire and the stock piled on the wire. The water flows into a funnel comprising a constant flow spout at the bottom of the funnel and a lateral tube in the bottom section of the funnel. A constant volume (24.2 ml) remains in the funnel between the constant flow spout (constant flow 8.83 ml/s) and the lateral tube. When water flows from the measuring tank into the funnel, part of the water flows out through the constant flow spout, a constant volume (24.2 ml) of water collects between the constant flow spout and the lateral tube and finally some of the water flows out through the lateral tube. In measuring freeness, this volume of water that has flown out through the lateral tube is measured in a measuring glass and this volume of water corresponds to freeness. The measuring is usually performed manually. The measuring method is arduous and sensitive to changes in temperature and consistency. The measuring method is also inaccurate with low CSF values.
Another known method for defining freeness is the Schopper-Riegler method disclosed in publication SCAN-C 19:65, Scandinavian pulp, paper and board, Testing committee, approved 1964, which is incorporated herein by reference. According to this standard method, a known quantity of paper stock is first poured on a spreader cone which is opened after a predefined period of time (5 s), the stock is filtered through a wire and a mat of fibre piling on the wire into a funnel with an orifice at the bottom and the side. Water flows out through the bottom orifice at a constant flow rate [1000 ml/(149 sxc2x11)≈6.71 ml/s]. A constant volume (7.5 ml-8.0 ml) remains between the bottom orifice and the side orifice. The volume of water flowing through the side orifice corresponds to freeness measured in SR units so that 0 ml corresponds to 100 SR units, 1000 ml corresponds to 0 SR units and thus one SR unit corresponds to 10 ml. The SR and CSF scales are reversed in relation to each other, i.e. the highest SR value corresponds to the lowest CSF value. This measuring is also usually performed manually. The measuring method is arduous and sensitive to changes in temperature and consistency. The SR measuring method is also inaccurate with extreme values.
Patent publication U.S. Pat. No. 2,602,325, which is incorporated herein by reference, discloses a method for measuring freeness similar to CSF measuring, in which freeness is determined by measuring the time that elapses when a predefined volume of liquid separates from a suspension in a measuring chamber. Alternatively, it is possible to measure the volume of liquid that separates from a suspension in a measuring chamber during a predefined time. This solution provides the disadvantage of being slow and susceptible to human error.
Freeness can also be measured with an automated measuring apparatus which resembles CSF measuring in principle, but uses vacuum. A sampling apparatus of the measuring apparatus takes a sample of paper stock, and water is added to the sample so that its consistency becomes approximately 3%. The temperature of the sample is measured and the sample is allowed to descend and settle on the wire for 5 seconds. After this, the water is drained from the tank through the wire using vacuum. After a certain period of time, the pressure difference caused by the cake piled on the wire is measured. After the pressure difference has been measured, the consistency of the paper stock is measured using the mass of the cake. Freeness can be calculated from the pressure difference. The disadvantage of this measuring method is that pattern making (calibration) is arduous and difficult, because the apparatus must be calibrated separately for each stock type.
Patent publication FI 80 342, which is incorporated herein by reference, also discloses an automated method and apparatus for defining the dry stuff, freeness and wire retention of pulp. The measuring of freeness is based on measuring the liquid level of a suspension in a measuring chamber as a function of time. The drainage rate is formed as a function of the mass of the dry stuff cake. The measuring requires a precise weighing machine so as to avoid errors in the freeness result. A weighing machine also increases the manufacturing costs of the apparatus.
Patent publication FI 51 133, which is incorporated herein by reference, discloses another automated method and apparatus for measuring freeness by directing pressurized water through a layer of stock and determining the freeness resistance by the volume of water that flows through during a certain period of time. The disadvantage of this measuring method, too, is that calibration is arduous and difficult, because the apparatus must be calibrated separately for each stock type. Pressurized measuring differs from standard measuring and the results are thus not comparable.
It is an object of the invention to solve the above-mentioned problems by implementing a method and an apparatus implementing the method. This object is achieved by a method described in the introduction and characterized in that in the method, when the flow starts at a time instant T0, the decrease in the suspension in the measuring chamber is measured as a function of time; a time instant T1 is searched for, at which the decrease in the suspension substantially corresponds to a previously known flow rate vc; and freeness F is determined as a function of the volume of suspension drained from the measuring chamber by the time instant T1.
The measuring apparatus of the invention is characterized in that it comprises a measuring sensor for measuring the drainage of a liquid from a measuring chamber through a wire or the like as a function of time; the measuring apparatus comprises an automatic data processing apparatus to which the sensor is made to feed its measuring data; the automatic data processing apparatus is adapted to search for a time instant T1 at which the drainage of the liquid from the measuring chamber substantially corresponds to a previously known flow rate vc; and the automatic data processing apparatus is adapted to determine freeness F as a function of the volume of suspension drained from the measuring chamber by the time instant T1.
The method and system of the invention provide several advantages. Measuring freeness becomes faster, more accurate and simple. Measuring does not require calibration and the measuring apparatus need not be set to the type of stock used in measuring. Further, repeatability improves, as the human factor involved in the measuring can be minimized. The measuring apparatus of the invention is also inexpensive and quick to take into use.